1. Field of the Invention
Aspects of the present invention relate to a polymer electrolyte membrane and a fuel cell including the same, and more particularly, to a polymer electrolyte membrane that is formed using a phosphoric acid substituted with an aliphatic hydrocarbon to attain excellent ion conductivity, heat resistance and liquid holding properties.
2. Description of the Related Art
A group of fuel cells form an energy generating system in which energy of a chemical reaction between oxygen and hydrogen contained in a hydrocarbon-based material (such as methanol, ethanol, or natural gas) is directly converted into an electrical energy. Fuel cells can be categorized into phosphoric acid type fuel cells, molten carbonate type fuel cells, solid oxide type fuel cells, polymer electrolyte membrane fuel cells (PEMFCs), alkali type fuel cells, and the like, according to the electrolyte that is used. These fuel cells operate based on the same principle, but have different fuels, different operating temperatures, different catalysts, different electrolytes, etc.
Among these fuel cells, the PEMFC has better energy output properties, a lower operating temperature, quicker initial operation, and a quicker response than the other fuel cells. Due to these advantages, the PEMFC has a wide range of applications, which include a portable power source for cars, an individual power source for homes or public buildings, and a small power source for electronic devices.
Conventionally, a PEMFC includes a polymer electrolyte membrane composed of a polymer electrolyte, such as a perfluoro sulfonate polymer (for example, NAFION produced by Dupont Inc.) that has a main chain of an alkylene fluoride and a side chain of vinyl ether fluoride terminated with a sulfonic acid group. In this case, it is noteworthy that the polymer electrolyte membrane attains high ionic conductivity by impregnation with a proper amount of water.
In order to prevent dehydration of the polymer electrolyte membrane of the PEMFC, the conventional PEMFC operates at 100° C. or less, for example, about 80° C. However, such a low temperature of 100° C. or less results in the following problems. A hydrogen-rich gas, which is a main fuel for the PEMFC, can be obtained by reforming an organic fuel, such as a natural gas or methanol. In this case, however, the hydrogen-rich gas contains CO as well as CO2 as a by-product. The CO poisons catalysts contained in a cathode and an anode of the PEMFC. When a catalyst is poisoned with CO, its electrochemical activity decreases significantly, and thus, the operation efficiency and lifetime of the PEMFC decrease significantly. In particular, it is noteworthy that the catalyst is more prone to poisoning when the operating temperature of the PEMFC is lower.
However, the temperature of the PEMFC can be easily controlled and when the operating temperature of the PEMFC is increased to about 150° C. or higher, the poisoning of the catalyst with CO can be prevented. As a result, a fuel reformer can be miniaturized and a cooling device can be simplified, and thus, the entire energy generating system of the PEMFC can be miniaturized. However, the conventional electrolyte membrane, that is, a polymer electrolyte such as the perfluoro sulfonate polymer (for example, NAFION produced by Dupont Inc.) that has a main chain of a alkylene fluoride and a side chain of vinyl ether fluoride terminated with a sulfonic acid group, experiences a significant drop in performance due to evaporation of moisture at a high temperature as described above. In addition, a polymer containing a sulfonic acid group fails to maintain its original form at about 120° C. or higher. As a result, the polymer electrolyte membrane formed using the perfluoro sulfonate polymer cannot act as an electrolyte membrane at high temperatures.
In order to solve this problem, non-humidified polymer electrolytes that can operate at high temperatures have been actively researched, and are based mainly on a polybenzimidazole (PBI)-phosphoric acid system that uses a phosphoric acid (H3PO4) as a proton conductor. The PBI-phosphoric acid system generally uses so called, 85% phosphoric acid containing 85% ortho-phosphoric acid. However, the ortho-phosphoric acid dissolves in water generated by the reaction between the hydrogen ions and oxygen molecules, and thus, the ionic conductivity of the electrolyte membrane decreases, and when the fuel cell operates for a long time at a high temperature, the polymer matrix dissolves in the phosphoric acid. In other words, when used at high temperature, a condensation reaction occurs among phosphoric acid molecules, thus forming a polyphosphoric acid. The formed polyphosphoric acid decreases the ionic conductivity and dissolves the polymer electrolyte membrane.
In order to solve this problem, the ortho-phosphoric acid can be replaced with a phenyl group (see U.S. Pat. No. 6,478,987). In this case, however, the acidity of a hydroxyl group of a phosphoric acid is decreased, and thus, the ionic conductivity decreases.
Accordingly, more research is required to develop a polymer electrolyte membrane that has heat resistance for maintaining the polymer electrolyte membrane at a high temperature for a long operating time, a liquid-holding property for reducing the leakage of the impregnated phosphoric acid, and excellent ionic conductivity.